Given that this is an unlimited tourney, I would venture to guess hardly any of the scores under par doesn't have a birdie on the 1st hole. Especially considering the extreme frustration the course already gives you, why not start out good, if you have the choice.
That being said everyone 2 under and below really didnt need #1 to have been birdied to be under par, it is just one hole in the grand scheme of everything afterall.